Tankers and other cargo ships are furnished with ballast tanks for maintaining the stability of the ship during traveling. The ballast tanks are typically filled with water when the ship is not loaded with oil or other cargo, and emptied when the ship is loaded. The buoyancy of the ship is thus re-adjusted to stabilize the ship.
Ballast water, essential as above for the safety of seagoing vessels, is normally the seawater taken in at the ports where the ships are unloaded of their cargo. It is estimated that, worldwide, as much as well over 10,000,000,000 tons of seawater is being used annually for ballasting ships.
Ballast water contains microorganisms and eggs of large or small organisms, which inhabited the ports where the water was taken in. As the ship travels the oceans, such microorganisms and eggs of large or small organisms contained in the ballast water are also transported to foreign countries.
Therefore, exotic species, which have not originally inhabited the destination ports, replace the native species, causing serious destruction of the ecosystems.
In light of these situations, a diplomatic conference at International Maritime Organization (IMO) made it obligatory to conduct regular tests for ballast water treatment equipment and the like, and this obligation will apply to ships constructed after 2009.
The ballast water performance standards of the Convention for the Control and Management of Ships' Ballast Water and Sediments (hereinafter referred to as the Convention) require that ships conducting ballast water management shall discharge:
TABLE 1Ballast WaterItemsQuality CriteriaSizePlankton10 Unit/ml10 to 50μmPlankton10 Unit/m350μm or moreEscherichia Coli250 cfu/100 ml0.5 to 3μmVibrio Cholerae 1 cfu/100 ml0.5 to 3μmGenus Enterococcus100 cfu/100 ml0.5 to 3μm
For these reasons, the development of a method for treating ballast water, which can reduce the concentration of microorganisms in ballast water to be discharged to approximately one hundredth of those in outer oceans, has become an urgent need. As a technique for disinfecting ballast water, a membrane treatment method using a membrane module has previously been proposed.
Because ballast water (seawater) is high in ionic concentration, when it is treated using a membrane module, scale may form on the membrane surface during long-term operation. If such scale, which mainly includes sodium chloride (NaCl) and calcium sulfate (CaSO4), adheres to or builds up on the membrane surface, the membranes may be clogged or broken. Moreover, if the scale incorporates organic matter therein to form a solid mass, it cannot be removed even by backwashing.
Therefore, in membrane treatment methods, it is important to reduce fouling in order to extend the membrane treatment time. To reduce fouling during membrane treatment, patent document 1 discloses a technique for reducing fouling by utilizing expansion of bubbles by reducing pressure.
Patent document 2 discloses a technique in which a constant current is applied between an anti-fouling conductive member and a counter electrode to thereby inhibit the adhesion of organisms on the surface of the anti-fouling conductive member of an aquatic structure or the like in contact with seawater, thereby preventing contamination of the conductive member.    Patent document 1: JP 2003-265935 A    Patent document 2: JP 2005-185206 A